Pho is lovely, but it's a breakfast/ morning snack, akin to saying that US cuisine is all about pancakes with golden syrup. For something a little out there nobody has mentioned yet would be to find somewhere that does old Imperial cuisine from Hue which sounds to me like a kind of Asian Versailles during Louis 15th. It's very pretty and super over the top.

More conventionally, you'll see a big difference between the complex flavours of cooking around Hanoi (which had more of a French influence) and in the south which is generally "cleaner". I've found the Hanoi flavours more to my personal taste (red wine stewed beef pho - mmm), but each to their own.